Fiber optic communication links are increasingly used in a variety of electric signal transmission applications ranging from cable television distribution, telecommunications, electromagnetic field sensors, and radar. A prime motivation for using these links is that the optical fiber transmission medium offers significant advantages such as high bandwidth, low loss per unit length, immunity to electromagnetic interference, and low weight. Unfortunately, many of these advantages are not realizable in practice because of limitations in the electrical-to-optical and optical-to-electrical conversion process.
To understand why this is so, consider one type of optical link wherein information is impressed upon a carrier light wave by modulating the current of a semiconductor diode laser. This process, referred to as direct modulation, is presently the most widely used technique for optical links.
However, direct modulation optical links have not been as widely accepted as their original proponents had expected. Where such links are to be used in place of a coaxial cable analog systems designers typically prefer links exhibiting efficient transfer of electrical power, low noise figure, and high dynamic range. Digital systems designers typically prefer high signal to noise ratio, low bit error rates, and efficient transfer of input current to output current, to enable high fan out. All of these features have heretofore been difficult to achieve with optical links.
For example, although the loss of the optic fiber itself may be less than 1 decibel per kilometer (dB/km), the electrical-optical-electrical conversion process typically results in a zero-length link insertion loss of 30 to 50 dB.
In another type of system, the laser is operated at a constant power level, and an optical modulator is coupled to the laser output. This so-called external modulation approach does have some advantages. For example, it allows the use of a laser that emits light at a fixed optical power level, thereby eliminating concern over the laser's linearity.
Known theory predicts that the ratio of the output electrical power to the input modulation signal power depends upon the square of the optical power available at the output of the modulator. See generally Gagliardi, R. M. and Karp, S., Optical Communications, (New York: John Wiley & Sons, 1976), pp. 141-155. However, insertion losses less than 30 dB, have not been observed in practical externally modulated optical links.
Thus, even when external modulation is used, existing optical links usually exhibit low transfer efficiency, whether transfer efficiency is defined as the ratio of link output electrical power to link input electrical power, as in the case of analog links intended to carry analog signals, or as the ratio of output current to input current, as in the case of digital links.
The transfer efficiency problem can be overcome somewhat by using an electronic amplifier at the receiver side of the link, or by using an avalanche photodiode or photomultiplier as the detector. In some applications, such as cable television, where a number of detectors are necessary, the expense of such an approach is undesirable and may be prohibitive, however.
Because the optical fiber medium itself provides increased efficiency in transferring light from the laser to the photodetector, it is quite common to reduce the amount of input laser power as much as possible. Operation at lower power levels is also encouraged by historical concerns, dating back to the design of early free space optical systems, that in the interest of efficiency, such systems should operate at low power levels. See Pratt, W. K., Laser Communications Systems, (New York: John Wiley & Sons, 1969), p. 16. Thus although higher power lasers have been used with free space optical communications systems and bulk-type, low efficiency modulators, there has seldom been an attempt to explore the use of higher power lasers in optical fiber links.
In certain applications, low optical power is used because of modulator stability problems in short wavelengths such as 100 microwatts (.mu.W) at a wavelength of 830 nanometers (nm) or because of limited power available from the laser, such as 1 milliwatt (mW) at 1300 nm.
Thus, many prior art optical fiber links typically operate at fairly low optical power levels--either because of historic reasons or because of practical considerations.
Existing rationale thus appears to be that there is little advantage to increasing the optical power in optic fiber links beyond the milliwatt power level, in spite of the theoretical teaching that link transfer efficiency improves with the square of optical power. An optical link exhibiting net electrical power gain has never been demonstrated.
Theoretical calculations of others, such as in Bulmer, C. H. and Burns, W. K., "Linear Interferometric Modulators in Ti:LiNbO3", IEEE Journal of Lightwave Technology, (New York: Institute of Electrical and Electronic Engineers), Vol. LT-2, No. 4, August 1984, pp. 512-521, imply that an improvement in link dynamic range will be observed with an increase in optical power. See also Cochran, S. R., "Low-Noise Receivers for Fiber-Optic Microwave Signal Transmission", IEEE Journal of Lightwave Technology, (New York: Institute of Electrical and Electronic Engineers), Vol. LT-6, No. 8, August 1988, pp. 1328-1337, wherein the sources of noise in an optical link receiver are discussed and mathematical relationships for their relative amplitudes are derived. However, neither of these references shows how to achieve shot-noise limited performance in an externally modulated optical communications system without using an electronic amplifier, avalanche photodiode detector, or photomultiplier.
What is needed is a way to improve electrical to electrical transfer efficiency of an optical link, as well as it other operating characteristics. The improvement should be such that optical links are attractive in a broad range of signal transmission applications, such as cable television distribution, telecommunications networks, and electromagnetic sensing.
The approach should be simpler and less costly than present techniques such as active electronic amplifiers or avalanche photodetectors.
It is also desirable to provide a mechanism for increasing the transfer efficiency of an externally modulated optical link by increasing sensitivity of the optical modulator, without necessarily decreasing the link's electrical bandwidth.